A telescopic gun sight or a riflescope is a device used to provide an accurate point of aim for firearms such as rifles, handguns and shotguns. It is used with other types of weapons such as bows and crossbows as well. A telescopic sight can dramatically improve the functionality of a firearm by providing the shooter with a simple yet highly accurate means for aiming at distant targets.
FIG. 1 shows a common riflescope design. With reference to this figure, a riflescope is comprised of an objective lens 1 which forms a first image of the target on the objective focal plane 4. The objective lens has an optical axis 50 which defines the optical axis of the riflescope. The first image produced by the objective lens is upside down and laterally reversed. An image-erecting means comprising a pair of lenses 3a and 3b relay this first image to the eyepiece focal plane 5 and form a second, upright and laterally correct image there. An eyepiece lens 2 takes this second image and produces a virtual magnified image for the shooter to see.
In riflescopes, the “point of aim” is usually designated by a reticle or cross hairs. In FIG. 1, the reticle 20 is mounted coplanar with the eyepiece focal plane 5. However, it can also be mounted at the objective focal plane 4. In either case, the shooter will see an image of the reticle superimposed on the image of the target. Telescopic sights are equipped with two control knobs for elevation (up-down) and windage (left-right) adjustments. These knobs mechanically move the reticle so that the sight's point of aim can be aligned with the firearm's “point of impact”. The amount of adjustment is usually very small and is measured in minutes of angle or MOA. For descriptions of the conventional mechanical methods commonly used to adjust the reticle within a telescopic sight see, for example, U.S. Pat. No. 2,955,512 issued Oct. 11, 1960 to Kollmorgen et al., U.S. Pat. No. 3,161,716 issued Dec. 16, 1964 to Burris et al., U.S. Pat. No. 5,463,495 issued Oct. 31, 1995 to Murg, and U.S. Pat. No. 5,615,487 issued Apr. 1, 1997 to Tomita.
An optical method for adjusting the point of aim in telescopic gun sights has been disclosed in U.S. Pat. No. 8,749,887 issued Jun. 10, 2014 to the present applicant. This patent teaches the use of two wedge prisms which move linearly along the optical axis to shift the image formed by the objective on the reticle. This novel optical method overcomes many of the drawbacks associated with the conventional mechanical methods.
A second optical method using counter-rotating (Risley) prisms has been disclosed in U.S. patent application Ser. No. 14/324,605 filed Jul. 7, 2014 by the present applicant.
The present application teaches an alternative optical method for adjusting the point of aim that utilizes tiltable wedge prisms. This method leads to a telescopic gun sight wherein the point of aim can be adjusted with extremely high precision. Implications and advantages of using tiltable prisms are discussed in detail in Section VI-G.